Whoa, no 6 year old should get anywhere near a "sit up". They serve no function at all and are very dangerous for the young spine. Even crunches are wrong for kids. Push ups serve zero function at 6 years old. Gymnastics at age 3-5 is also of low value in regards to future development. Little ones that age should do simple playground play, basic running, jumping, climbing, hopping, simple games of catch. Of course they can swat a foam or low compression tennis ball around. They do not need any supplemental things like gymnastics or body weight exercises yet. Okay maybe some basic balancing walking on a curb or something....but thats more playground play than the need for true gymnastics. An older kid around 7-9, sure, a few basic gymnastics classes would be good for helping them work on their spatial awareness and balance. Please read a book such as Children and Sports Training by Jozef Drabik which outlines proper training for young children. It explains the sensitive periods where certain components of fitness should be worked on....and the stages where they are worthless. And no 14 year old should be doing any weight exercises that compress the body in any way....no overhead lifts with actual weight. No deadlifts. At that age they can work with resistance bands and medicine balls, always exercising while on their feet. No need for machines or free weights yet. They can emulate most exercises safely using bands and medicine balls. Once the boys hit about 16, the actual real weight training will be both safe and beneficial enough to be worth it. And even at that age, it must be supervised by a true expert trainer.